The present invention relates generally to a molded article and, more particularly, to method of injection molding an article having a modified surface for enhanced adhesion.
Polyolefins have many desirable physical properties, and are often used to create molded articles. These molded articles have a wide range of applications, such as automotive interior panels and decorative components. Polyolefins have low surface energy leading to issues of bonding with surface coatings, such as adhesives or paint. Therefore, surface modification is typically implemented to increase the surface energy of a polyolefin-based article and to improve bonding between the article and adhesives or other coatings.
Unmodified polyolefins typically have a surface energy of approximately 30 dynes/cm, but a surface energy of 38-50 dynes/cm (or higher) is desired to more effectively bond adhesives or other coatings to the polyolefin article. Thus, in many situations, polyolefin articles often undergo surface modification after the articles are formed. This process may be termed post-polymerization surface modification. Such post-polymerization surface modifications include additional manufacturing steps and tooling part complexity.
Corona discharge is a post-polymerization surface modification resulting in oxidation of the polyolefin surface. During corona discharge, an electrical source generates a current across an area where treatment is to be applied. The emission of electrons in the air near the surface of the polyolefin article generates ions and free radicals which in turn oxidize the surface of the article. This oxidation typically occurs 2 nanometers (nm) past the article surface, and it typically raises the surface energy of the article to a level where adhesion is possible (38-50 dynes/cm). The corona discharge method may be undesirable for various reasons. For example, due to the relatively shallow depth of oxidation, the positive effects of corona discharge may be reversed over time if the article is exposed to the environment. Additionally, the corona discharge method is often less effective on curved surfaces than it is on flat surfaces.
Plasma discharge is another post-polymerization surface modification resulting in oxidation of the polyolefin article. Plasma discharge is a method similar to corona discharge except that the oxidation process takes place within a vacuum. The method used and the inherent problems associated with plasma discharge are similar to those associated with corona discharge. This oxidation typically occurs 2 nm deep past the article surface, leading to possible reversal of the effects over time. Although the plasma discharge method is more effective with curved or irregular shaped parts than the corona discharge method, the plasma discharge method requires additional manufacturing components and costs associated with the need for sealing the article within a vacuum.
Flame treatment is a post-polymerization surface modification resulting in oxidation of the surface of the polyolefin article. During the method of flame treatment, gas burners generate a flame that causes oxidation in the surface of the molded article. The extent that the molded article undergoes oxidation is generally controlled by two variables, the air/gas ratio and the distance between the flame and the article. These variables often cause inconsistent oxidation in the surface of the article, particularly with curved or irregular shaped parts. Also, the oxidation typically occurs 2 nm deep past the article surface, which leads to possible reversal of the effects over time. Additionally, air currents may interfere with the flame treatment. Furthermore, other general problems associated with fire hazards and working with an open flame may occur.
The chemical treatment method is a method of post-polymerization surface modification of the polyolefin article resulting in surface oxidation. Chemical methods typically used an aqueous potassium dichromate-sulfuric acid solution in order to oxidize the surface of the article. However, the hazardous nature of this solution creates high waste disposal costs and undesirable health hazards. Additionally, the treatment may result in degradation of the polymer surface.
Increasing surface roughness is a method of post-polymerization surface modification of the article without oxidation. The article surface roughness may be increased by sanding or roughing the article after it is formed, thereby increasing the surface area of the article. Additionally, the inner surface of the mold may undergo a special tooling process in order to create a rough surface of the article during part formation. However, neither technique of increasing surface roughness effectively increases adhesion to the article despite the increase in surface area from surface roughing. Additionally, the special tooling required for roughing the inner surface of the mold increases manufacturing complexity.
Solvent base primers may be used for post-polymerization surface modification resulting in increased polarization of the article surface. The solvent based primer typically used in this method is a low solid (˜5%) solution of chlorinated polyolefin dissolved in a high Kauri Butenol solvent or compounded with film forming resins and additives. The solvent aids in penetration or diffusion of the primer into the surface of the plastic. After the solvent evaporates, the polar halogenated polymer is anchored into the article surface. This method requires the steps of spraying or dipping the article into the above described solvent based primer as well as the additional steps of drying the article. Moreover, this method requires further manufacturing components necessary to promote drying because the solvent based primer typically requires drying conditions higher than room temperature. Such manufacturing components may include an oven for drying the solvent based primer or a dust free, dry staging area for evaporation of the same. Furthermore, the solvents used are toluene, xylene, or other flammable materials which require special handling and permits due to their high volatile organic compounds (V.O.C.).
In addition to having low surface energy, unmodified polyolefins have low conductivity. Conductive articles are often painted or otherwise coated using an electrostatic operation wherein particles of the coating substance are given an electrostatic charge and sprayed onto an electrically grounded article. The charged particles and the grounded article form a magnetic attraction, promoting an evenly-coated article. However, if the article has low conductivity, the magnetic force between the two components is relatively weak or nonexistent. Polyolefin particles have low conductivity, and therefore are currently modified to increase surface conductivity via methods similar to those described above with respect to surface modification for increasing surface energy. More specifically, methods of improving surface conductivity require post-polymerization surface modification, and result in additional manufacturing steps and manufacturing tooling complexity.
Therefore, it is desirable to reduce the process complexity and reduce the manufacturing part complexity for surface modification of polyolefin articles.